Application Note | Case Hardness Profile
Hardness Distribution of Hardened Metal Using Nanoindentation
Mapping Hardness and Modulus from the Treated Surface into the Core
Research & Experimental Testing
Duanjie Li
Visual Design & Editorial
Andrew Shore
Introduction
Surface hardening is a process that hardens the surface of a metal object while maintaining the softness and toughness of the core metal deeper underneath. The combination of a hard surface and soft interior metal is important for achieving superior resistance to breakage upon impact such as high stress and fatigue, while enhancing the wear protection of the metal surface. Such a unique property is required in parts for numerous industrial applications, such as a cam or ring gear, anti-friction bearings or shafts, turbines and automotive components1.
Importance of Nanoindentation Mapping for Surface Hardened Metal
Surface hardening can be categorized into two approaches in general: (1) buildup or addition of a new layer; (2) surface and subsurface modification without an additional layer. Thin films, coatings and overlayers are deposited to provide an enhanced surface in the first case, where the homogeneity and interfacial bond strength play an important role. In the second case of direct surface modification such as carburizing, nitriding and peening, there is usually no clear boundary between the treated surface and the metal beneath, which makes it challenging to determine the thickness of the surface layer. Nanoindentation provides extremely localized precise hardness and elastic modulus measurements, making it an ideal tool for hardness mapping to determine the quality, homogeneity and thickness of the surface hardened layer.
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Measurement Objective
In this application, the NANOVEA Mechanical Tester in nanoindentation mode is used to measure the hardness distribution of the hardened surface on the cross section of a surface hardened metal. We would like to showcase the capacity of the NANOVEA Mechanical Tester in performing nanoindentation mapping with high precision and reproducibility.
NANOVEA PB1000 Large Platform
Mechanical Tester
Test Conditions
The hardness, H, and elastic modulus, E, of 120 points along the distance from the treated metal surface on the cross section were measured, spaced 50 µm apart for a total distance of 6 mm, as shown in Fig. 2. The test parameters are summarized in Table 1.
| Parameter | Value |
|---|---|
| Maximum force (mN) | 100 |
| Loading rate (mN/min) | 200 |
| Unloading rate (mN/min) | 200 |
| Creep (s) | 5 |
| Indenter type | Berkovich diamond tip |
Table 1: Test conditions of the hardness measurements.
Results and Discussion
Surface hardening enhances the surface hardness while maintaining the soft, tough nature of the interior metal. Hardness distribution measurement along the thickness of the treated surface ensures the quality and homogeneity of the surface treatment. The H and E values as a function of the distance from the treated metal surface are plotted in Fig. 3. It shows that the metal sample possesses the highest H value of ~9 GPa at the surface. The H value progressively decreases to a value of ~5.5 GPa until a distance of 3 mm from the surface is reached, after which the metal has a consistent hardness of 5.5 ± 0.5 GPa. Despite the hardness enhancement on the surface, the elastic modulus maintains a homogeneous value of 225 ± 12 GPa for the surface and interior metal. Fig. 4 shows the load-displacement curves measured at the surface and interior of the metal.
Conclusion
In conclusion, we have shown that the NANOVEA Mechanical Tester in nanoindentation mode provides reliable measurement of hardness and elastic modulus distribution on a surface hardened metal sample. It enables users to assess the quality of the surface treatment and to determine the thickness of the surface layer or coating, which is critical in R&D and quality control for coatings in a variety of industries, such as cutting tools, aerospace, automotive and many others.
The Nano, Micro or Macro modules of the NANOVEA Mechanical Tester all include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest and most user friendly range of testing available in a single system. In addition, optional 3D non-contact profiler and AFM Module are available for high resolution 3D imaging of indentation, scratch and wear track in addition to other surface measurements such as roughness.
References
1. Schneider, M. J.; Chatterjee, M. S., Introduction to Surface Hardening of Steels, ASM Handbook, Volume 4A, Steel Heat Treating Fundamentals and Processes.
Frequently Asked Questions About Hardness Depth Profiling
Why measure a hardness depth profile?
Surface hardening gives a hard, wear-resistant surface over a soft, tough core. In treatments such as carburizing, nitriding and peening there is no clear boundary between the treated layer and the base metal, so a hardness profile is the way to judge the layer’s quality, homogeneity and thickness.
How was the profile measured?
120 nanoindentations spaced 50 µm apart along 6 mm of the cross section, from the treated surface inward, with a Berkovich diamond tip to 100 mN.
How deep did the hardening go?
Hardness was highest, about 9 GPa, at the surface and decreased progressively to about 5.5 GPa at 3 mm, beyond which it stayed at 5.5 ± 0.5 GPa.
Did the elastic modulus change?
No. The elastic modulus stayed homogeneous at 225 ± 12 GPa from the surface to the interior.
Why use nanoindentation rather than conventional hardness?
It gives extremely localized, precise hardness and elastic modulus values at closely spaced points, which suits mapping a gradual hardness gradient.
Where is this used?
In R&D and quality control of surface treatments and coatings for parts such as cams, ring gears, bearings, shafts, turbines, cutting tools and automotive components.
